Tear string for shrink film packages

ABSTRACT

A tear string having particular utility in connection with packages overwrapped with heat sealable shrink film comprises an elastic core, one or more textile covers wrapped around the core and an outer coating of a fused thermoplastic synthetic resin. The core is in tension and, additionally, the outer coating may be axially oriented. Thus, when the string is heated to the thermoplastic temperature of the resin coating, the stored tensile forces in the core are released and tend to make the string contract toward its original dimensions. This effect is enhanced if the coating is axially oriented, due to the heat shrinking phenomenon. Such a tear string has various advantages in shrink film wrapped packages.

United States Patent Forman [72] Inventor: Harold M. For-man, 151 East 10th St.,

Conshohocken, Pa. 19428 [22] Filed: Nov. 5, 1970 211 App]. No; 87,238

[52] U.S.C|. ....l6l/175, 206/D1G. 4, 229/518 [51] ..B65d 17/20 [58] FieldofSearch ..16l/99-101, 172, 161/175, 242, 402; 57/144, 149, 152; 206/016. 4; 229/66, 86, 51 R, 51 S, 51 AS, 51 TS [56] References Cited UNITED STATES PATENTS 2,840,882 7/1958- Smith ..16l/l75 3,352,480 11/1967 Forman. ..229/51S 3,456,780 7/1969 Forman.... ..229/51 S 4/1970 Aleixo ..l6l/l75 Wagner, Jr. et al ..57/ l 44 Primary Examiner--Robert F. Burnett Assistant Examiner-Linda C. Koeckert Attorney-Barry Moyerrnan [57] ABSTRACT A tear string having particular utility in connection with packages overwrapped with heat scalable shrink film comprises an elastic core, one or more textile covers wrapped around the core and an outer coating of a fused thermoplastic synthetic resin. The core is in tension and, additionally, the outer coating may be axially oriented. Thus, when the string is heated to the thermoplastic temperature of the resin coating, the stored tensile forces in the core are released and tend to make the string contract toward its original dimensions. This effect is enhanced if the coating is axially oriented, due to the heat shrinking phenomenon. Such a tear string has various advantages in shrink film wrapped packages.

10 Claims, 3 Drawing Figures PATENTEDJUN B1912 JNVEN r012.

Harold/V1 Forman a Q mm mm mm v NM QM m mt 7///////// JON lwM| 1 a 9 WWW M MAT v m Q Q\ QN rw\\w 7 I10! a w R l TEAR STRING FOR SHRINK FILM PACKAGES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to strand structures. More particularly, it relates to covered or wrapped strands having an elastic core and coated with a thermoplastic material.

2. Prior Art The use of tear strings in connection with shrink film packages is exemplified in my own prior U. S. Pats. Nos. 3,352,480; 3,456,780 and 3,499,260. These patents disclose a tear string incorporated in a packagewhich comprises an item overwrapped with-shrink film (i.e., axially oriented thermoplastic film). When the package is heated, the film shrinks into tensioned juxtaposition with the item. Suitable shrink films and their characteristics are well known and are described, for example, on pages 196-202 of Modem Packaging, Encyclopedia Issue, Volume 43 No. 7A, July 1970.

A tear string, which may be a filament or aribbon, is incorporated within the package between the item and the film. At least one end is brought to the exterior of the package through a sealed (e.g. heat sealed) edge. This end is grasped and used to open the package by progressive tearing of the film.

While simple strings, filaments and ribbons can be used, they do not meet certain desiderata. For example, in connection with the apparatus of U. S. Pat. No. 3,499,260 it is desirable. that the tear string have a relatively high density so that it can be easily moved and positioned by fast moving machine elements. Further, since it is necessary in all of these packages to retain a portion of the tear strip within the package, it

' would be most desirable to be able to anchor the cutter or tear strip withouttotal reliance on either a mechanical anchoring means or frictional detention forces. Finally, during the shrinking of the film (e.g. as .-in a heating tunnel) it would be desirable to avoid buckling or sagging of the string due to the differential effect of the heat on it vis-a-vis the overwrapping film. Were the string always to remain taut and present a neat linear appearance it would have a better appearance than were slack to appear in the finished package.

SUMMARY OF THE INVENTION I The invention comprises a tear string which utilizes, as a starting material, an elastic core (A) with one or more textile coverings (B, B, etc.) wrapped about it. Such materials are conventional items of commerce, often referred to as covered elastic thread. They may be supplied, for example, with a core of rubber or spandex, one wrapping of cotton and another outer wrapping of nylon. The core may be multi-stranded or a single elastic strand. Several such'materials are described in U. S. Pat. No. 3,091,856 albeit used in another application. Generally, any covered elastic thread is suitable which is capable of being stretched to at least 110 percent of its original length and substantially fully recovering upon release of ten- 'sion.

films and which can be axially oriented. The coating (C) keeps the core of the tear string tensioned and elongated compared to its non-tensioned length. The tear string stays in this elongated condition during storage and while it is being incorporated into a package wherein shrink film is used as an overwrap. During the sealing of an edge of the shrink film package (such as that shown in FIGS. 1 and 2 of U. S. Pat. No. 3,456,780) through which edge the tear string passes, the tear string is welded to the axially oriented film by the same impulse wire which creates the seal and is thus fused or welded in a "sandwich" of shrinkfilm and is anchored thereby. Further, when the package is then subjected to heat as by being passed through a shrink tunnel) the thermoplastic coating temporarily softens the stored tensile forces in the core are permitted to tend to restore the tear string to its original length. The efi'ect, within a package of the type described, is to eliminate any slack from the tear stringand make it taut and linear. This latter restorative action can be enhanced by coating the core so that the thermoplastic material is axially oriented, just as shrink film is. Then, the heat in the shrink tunnel will cause the coating to shrink and thus reinforce the restorative forces within the core. Finally, by selection of a coating which is compatible with the overwrapping film and the use of appropriate tunnel temperatures, one can achieve a tacking of the taut tear string to the film itself wherever the two are in contact which increases the positive directional nature of the progressive tearing when the package is opened.

Accordingly, it is an object of the invention to provide a tear string for shrinkpackages which will weld to the shrink film, particularly between heat sealed edges thereof, and which will decrease in length whenthe package is exposed to conditions adapted to shrink the overwrapping film into heat shrunk tensioned juxtaposition with the item wrapped therein.

It is another object of the invention to provide a tear string having a wrapped elastic core in which tensile forces are stored and which has an "outer coating of thermoplastic material which, when heated, will activate length restoring forces including, if desired, forces caused by molecular orientation within the coating itself.

These and other objects of the invention will be apparent to those skilled in the art from a consideration of the description of exemplary embodiments of the invention which follows. Neither that description nor the foregoing abstract and summary is intended to limit or otherwise restrict the scope of the invention. The abstract and summary have been inserted primarily as tools for reader orientation and information retrieval.

BRIEF DESCRIPTION or THE. DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates the tear string of the invention. As shown it comprises an elastic core A which may be made of rubber, spandex or any other flexible resilient material. The core must be capable of at least 10 percent elongation and, preferably, about percent elongation under tension without destroying its ability to return to its original length or substantially its original length. This core may be provided with one or more textile wrappings. As shown, there is an inner cover of cotton B, wrapped about the core in one direction and an outer cover of nylon B, wrapped about the inner cover in the other direction. The thermoplastic material C encapsulates the previously described core and wrappings. The presence or absence of tensile forces in the tear string, as discussed above, is not amenable to illustration.

FIG. 2 illustrates one method of making tear string of the invention. As shown therein, a take-up or capstan 10 pulls a supply of covered elastic thread 1 1 off a feed reel or spool 12. The thread is the untreated material previously described. The thread passes over and under a plurality of tensioner fingers l3, idlers l4 and 15 and into a temperature controlled bath, generally 16. In the bath is a warm solution-26 of the thertemperature, etc. which is references.

3 moplas tic material being utilized as coating C. For example, the bath may contain a solution of polyvinyl chloride in methyl ethyl ketone MEK) held at a temperature of about 190 F.

. Wherepolyethylene is the coating material, toluene is into the bath. The doctor 23 also assures even coating and absence of excess. Between idlers and 21 the now coated thread is allowed to air dry anda catenary about 60 feet in length is found to provide adequate drying time when coating with polyvinylchloride in MEK, using a capstan having an effective diameter of about 1 foot, roating at from 75-200 r.p.m.

Following the air drying, the coating is tack free and is then passed through oven 24 which operates at the fusion temperature of the particular resin or thereabout (e.g. 270 F for PVC). Following the fusion, the tear string is air-cooled between idler 22 and take-up 10 The. action may be enhanced by a fan 25. The'co'ntrols such as bath heaters, thermostats,

agitators-etc. are conventional and a solvent recovery system r may be utilized if desired. 'The take-up reel is preferably driven by a variable speed motor with speed being controlled as a function of tension in the string which may be detected by conventional sensors. Tension can alternatively be created by positively rotating the feed roll 12 at a lower speed than take- The specific nature of the bath, its temperature and solvent as well as the temperature of the oven 24 are all functions of the particular thermoplastic used and can readily be determined by those skilled in the art from data on solubility, fusion found in standard texts and FIG. 3 is generally similar and,accordingly, the parts and their function are identically numbered except where different. The differences have to do with the position and function of the tensioners. As shown in the FIGURE, a plurality of tension fingers are provided on the feed end of the oven 24 anda plurality of fingers 31 are also provided downstream of the discharge end, after the air cool. The coated thread is I guided through the oven by idlers 32 and 33. With this arrangement, the thread is not tensioned until after idler 21, at which time the coating C has air dried to tack-free condition.

However, it is elongated at the time it is fused in oven 24 and stays elongated until it sets during subsequent cooling. Due to the nature of the coating material (e.g. low density polyethylene, polyvinyl chloride, etc.) this stretching during fusion axially orients the molecules in the direction of tension a 4 in the same way that film itself is axially oriented. Thus, on reheating (as in a shrink tunnel) the coating will tend to shrink and thus help restore the string to its untensioned length.

Tear strings made in this manner havethe advantages indicated above. They-can be anchored and welded into heat sealed edges of shrink film overwrapped packages- When the package is heated, the tear string reduces in length to tautly extend about the overwrapped item and, at appropriate tunnel temperatures (e.g. about 300 F) will tack to the film itself. Further, its weight due to the addition of the coating is such as to be easily manipulable during the packaging operation.

While the invention has been illustrated and described in detail, such description is not exhaustive of the possible variants and equivalents. Since it will be apparent to those skilled in the art that numerous changes and modifications may be made, it is not intended that the invention be construed as limited to the specific embodiments discussed above. Rather, its scope is to be limited only bya reasonable interpretation of the appended claims.

Iclaim: l. A tear. string comprising an elastic core whichcontains stored tensile forces and is, consequently, elongated compared to its non-tensioned length; at least one textile covering wrapped about said core; and a coating layer of fused thermoplastic heat sealing material encapsulating said covering, which layer keeps said string in tensioned, elongated configuration until said coating layer is heated to its thermoplastic temperature at which time said forcesstring to its original length.

2. The string of claim 1 wherein said thermoplastic material comprises polyvinyl chloride.

3. The string of claim 1 wherein said thermoplastic material comprises polyethylene.

4. The string of claim 1 wherein said elastic core is rubber.

5. The string of claim 4 wherein said thermoplastic material comprises polyvinyl chloride.

6. The string of claim 4 wherein said thermoplastic material comprises polyethylene.

7. The string of claim 1 wherein said thermoplastic material is axially oriented along the longitudinal axis of said core and heating thereof creates shrink forces which reinforce the length restorative tensile forces in the core.

8. The string of claim 1 wherein said core is rubber and which further includes an inner textile cover wrapped thereabout in one direction and an outer textile cover wrapped over said inner cover in a different direction to minimize unraveling and also to insure that said core will be covered in its elongated position.

9. The string of claim 8 wherein said thermoplastic material comprises olyvinyl chloride.

10. The string of claim 8 wherein said thermoplastic material comprises polyethylene; v

. a: a: a a:

tend to restore said 

2. The string of claim 1 wherein said thermoplastic material comprises polyvinyl chloride.
 3. The string of claim 1 wherein said thermoplastic material comprises polyethylene.
 4. The string of claim 1 wherein said elastic core is rubber.
 5. The string of claim 4 wherein said thermoplastic material comprises polyvinyl chloride.
 6. The string of claim 4 wherein said thermoplastic material comprises polyethylene.
 7. The string of claim 1 wherein said thermoplastic material is axially oriented along the longitudinal axis of said core and heating thereof creates shrink forces which reinforce the length restorative tensile forces in the core.
 8. The string of claim 1 wherein said core is rubber and which further includes an inner textile cover wrapped thereabout in one direction and an outer textile cover wrapped over said inner cover in a different direction to minimize unraveling and also to insure that said core will be covered in its elongated position.
 9. The string of claim 8 wherein said thermoplastic material comprises polyvinyl chloride.
 10. The string of claim 8 wherein said thermoplastic material comprises polyethylene. 